1. Field of the Invention
This invention is concerned with a fluid catalytic cracking process (FCC) for the production of diesel oil from vegetable oils. More specifically, the invention refers to the production of diesel oil in refineries which have two or more FCC reactors. At least one reactor processes conventional feedstocks (petroleum or residue), while another one, simultaneously, processes vegetable oils in operational conditions proper to produce a diesel oil better quality, that is, with cetane number higher than 40 and sulfur free.
2. Description of the Prior Art
Since the middle of the last century, a large number of researches seek alternative technologies to produce fuel from renewable sources or industrial wasting. The transesterification reaction or alcoholisis appeared as a novel procedure, significantly advantageous, to allow the obtention of fuels from triglycerides which are present, for instance, in vegetable oils. References regarding this subject can be found in the European publication EP 00127104. One employs methanol, or alternatively ethanol, to yield long chain esters and glycerin, as it shows the chemical equation below:

The U.S. Pat. No. 4,695,411 describes a procedure in which the main target is to provide a profitable process employing hydrated alcohol to obtain highly pure esters mixture. The process is applicable to either animal or vegetable oils, including seeds oils.
Similarly, the U.S. Pat. No. 4,164,506 and the U.S. Pat. No. 4,698,186 describe processes in which one use acidic catalysts to esterify triglycerides in two stages.
The non-alcaline catalysts, as described in the U.S. Pat. No. 5,525,126, show an additional advantage since they do not catalyze the formation of soap and, therefore, they do not allow the triglycerides pre-esterification.
In the U.S. Patent Application No. 2005/0113588A1, a fuel preparation method, from several oils or combinations among them, employs two reactors and one heterogeneous catalyst in order to accomplish the alcoholisis. The catalyst is made up of a mixture of zinc and aluminum oxides.
In the publication WO 03/093400, one employs phosphorous compounds as polymerization inhibitors and iron or copper compounds as reduction agents, in order to adjust oxidative cracking reactions with ozone. After successive filtering and cracking steps, one obtains a lighter compounds fraction which exhibit excellent properties for diesel oil use.
The transesterification with methanol or ethanol, however, presents somewhat constraints. The necessity to transport and to handle large amounts of such inputs requires excessively high investment for the assembling and maintaining safe plants, mainly due to the its effective risk of intoxication and fire. Moreover, the burning of fuel produced from glyceride alcoholic transesterification generates a considerable amount of formaldehyde, acrolein and benzene, which besides pollutants they cause damages to pistons and engines.
The state-of-art shows that in studies cited in the technical literature about vegetable oils processing in fluid catalytic cracking units (UFCCs), the vegetable oil is always mixed to the conventional feedstocks. The literature does not mention studies focused on the diesel production directly from vegetable oil. (BUCHSBAUM, A.; HUTTER, K.; DANZINGER, F; LICHTSCHEIDL, J. The Challenge of the Biofuels Directive for a European Refinery, ERTC 9th Annual Meeting, Praga, 2004).
The Brazilian Patent No. PI 8304794, approaches the high octane gasoline production. One introduces the vegetable oil and the conventional FCC feedstock (petroleum) together in the FCC reactor. The gasoline produced is of excellent quality, because it is highly aromatic and sulfur free.
In the UFCCs, the difficulty of producing good quality diesel oil, from the mixture of vegetable oil and conventional feedstock, is due to the very high reaction temperatures, never lower than 490° C. Moreover, the vegetable oil volume available for diesel oil production is too much small compared to the petroleum volume processed nowadays. Compared to the transesterification process, investment costs used to be too high that they do the assembling of an UFCC unfeasible for processing exclusively vegetable oil. Hence, one could introduce this alternative route, for diesel production at the refinery's UFCC, where two reactors already exist.
Although the processing at temperatures above 490° C. brings about the drop of fuel oil formation, which is of little value-added, it favors a large number of hydrogen transfer reactions. Thereby, it yields an expressive amount of aromatic compounds, in spite of such compounds are absent in the vegetable oil of the feedstock. However, the diesel oil produced at high temperatures is not of good quality, inasmuch as its cetane number is low. Low temperatures do not vaporize the feedstock utterly in the FCC reactor. When the feedstock is not vaporized utterly just in the reactor feeding inlet, the catalytic selectivity drops, because the catalyst pores get blocked. Thereby, most of the reactions takes place on the particles surface, seeing that in liquid phase the feedstock do not diffuse toward the catalyst's micropores and do not reach the core active sites.